In small carburetors designed for use with low displacement gasoline fueled engines, such as used on chain saws, weed whips, lawn mowers, garden tractors and other small lawn, garden, and forestry portable appliances, manually operated choke and throttle controls are typical provided and often hand cranking is employed for starting the engine. Prior to the late 1970s, chain saws equipped with such choke and throttle controls often involved a basic starting sequence which left much to be desired. First the choke valve was fully closed to its start position, and then the starter rope was pulled until the engine fired. The closed choke valve usually caused the engine to immediately die at this first firing due to over-enrichment of the air fuel (A/F) mixture. This is commonly referred to as a false start. At this point the choke valve had to be opened. Then the starter rope was pulled again until the engine finally began running.
This starting sequence was subsequently improved by adding another start-up control to the chain saw whereby the throttle valve could be held at a partly opened position, known as fast idle position. This generally avoided false starts due to the increased air flow permitted past the throttle valve.
In order to avoid the need for three separate manually operated controls, namely, a throttle control, a choke control and fast idle start control, Johansson U.S. Pat. No. 4,123,480, issued Oct. 31, 1978 (which is incorporated herein by reference), disclosed an improved chain saw engine control mechanism. The automatic fast idle setting mechanism of the Johansson U.S. Pat. No. 4,123,480 is shown herein in FIGS. 1, 2 and 3 which correspond respectively to FIGS. 1, 3 and 4 of the '480 patent. The direction of air-flow through the carburetor throat is indicated by the arrow labeled "A" in these views, as well as in all other views in the drawings herein. For convenience, the reference numerals employed in FIGS. 1, 2 and 3 are those employed in '480 patent, to which further reference may be made for the details of the construction and operation of the same.
In the '480 patent a fast idle secondary lever 9 is pivoted on the choke valve shaft 11 and is operable to engage a tang of a latch arm of a throttle lever 4 fixed on the throttle valve shaft 2 to cause the throttle valve 1 to open to a predetermined angle corresponding to the fast idle position (FIG. 2). With this arrangement, the operator need only operate a single start-up control, namely the choke valve control (not shown) coupled to the choke shaft control lever 12 in order to set the throttle 1 in fast idle condition. Thus when the operator moves the choke control to swing the choke valve 10, via lever 12 from fully open position (FIG. 1) to its fully closed start position (FIG. 2), the pivotal motion of choke shaft control lever 12, via a coupling tang 14 on the adjacent fast idle lever 9, pivots fast idle lever 9 and causes its notch 8 to latch engage and hold the throttle lever latch arm tang 7, thereby automatically setting the fast idle latch mechanism. The bias of the respective choke and throttle shaft return springs 15 and 3 also provide the yieldable latch closing forces.
Then, if the chain saw engine experiences a false start, the choke lever 12 may be moved to the open position (FIG. 3) without thereby moving the fast idle lever 9 so that it remains engaged with the throttle lever 4 to retain the throttle valve 1 in the fast idle position. Once the chain saw engine starts, the operator simply depresses the throttle control trigger 6 to open the throttle valve 1. This pivots the throttle shaft lever 4, thereby causing it to disengage the fast idle lever 9 and thus cause release of the latch. If the choke valve 10 was still in the closed position at this point, the choke biasing spring 15, acting through the fast idle lever 9 and tang 14 coupling it to the choke lever, would automatically cause the choke valve 10 to be returned to full open position upon such unlatching of the fast idle lever 9 from the throttle lever 4 (FIG. 1).
One of the disadvantages of this '480 patent design is its failure in practice when mass produced to insure complete and/or consistent closure of the choke valve 10 when setting the fast idle latch starting system. The specific problem has been found to be due to the choke valve sometimes not completely closing even though the operator has fully engaged the choke control to indicated start position. Further, it has been found that this problem is due to a stack up of normal manufacturing tolerances in the parts as manufactured for assembly into the fast idle latch mechanism.
Such manufacturing tolerances are, of course, necessary to set up minimum dimensional range limits or allowances to accommodate normal manufacturing equipment capabilities at acceptable manufacturing cost levels. This is a particular problem in producing carburetors for engines for chain saws, lawn mowers, clearing saws, weed whips, etc. that require very low manufacturing cost due to the low retail price of such consumer products. The problem is compounded due to the small size of the carburetors for such small engines, and the corresponding minuscule size of the choke and throttle parts involved in the carburetor mechanisms. These factors make it particularly difficult to reduce manufacturing tolerance allowances in order to reduce the adverse effects of unavoidable manufacturing dimensional variations in such tiny parts when assembled for operation in the mechanism.
Thus, in the case of the incomplete and/or inconsistent closure of the choke valve in the operation of the fast idle starting system of the '480 patent arrangement, it has been found that a shift in tolerances for all parts (tolerance stack-up) in the latch mechanism to one end limit will render the choke valve incapable of reaching the fully closed position. This prevents, or at least hinders engine starting. On the other hand, a tolerance shift in all of these parts to the opposite end limit will cause the fast idle lever to fail to even engage with the throttle lever, so that no "latch up" action occurs. This results in a loss of function of the entire choke throttle fast idle system.
The culprit in this problem has been found to be the push coupling via tang 14 between the choke lever 12 and fast idle lever 9. This dictates that the actual position of choke valve 10 when swung toward closed position will be controlled by the latched up position of fast idle lever 9 when the engaged throttle lever latch tang 7 and idle lever notch 8 of the latch system (if indeed engaged) swing slightly back to their spring held, stable, latched position after manipulating forces are removed from the manual controls of the appliance, as will be explained and seen in more detail hereinafter in conjunction with FIGS. 8-13. It is also noted that the '480 patent states (but does not illustrate or explain) at column 4, line 60-65 that it is possible to arrange the '480 mechanism in such a way that it can arrest the throttle valve and the choke valve in several different combinations of positions by designing the fast idle lever and/or the throttle lever with several recesses and projections respectively. Although this general statement is obviously ambiguous and unclear, this variation may be intended for the purpose of somehow providing sequential engine operational stages such as part-open choke valve positions often used for certain appliance applications, or for engines for which such adaptations may be suitable or desirable to satisfy differing engine operational mode requirements to suit the load and conditions of use of the appliance.
Another prior art solution to the problem of achieving automatic fast idle setting of the throttle valve is found in Hermle U.S. Pat. No. 5,200,118, issued Apr. 6, 1993 and assigned to Walbro Corporation of Cass City, Mich., assignee of record herein. A fast idle throttle latch system with automatic release in accordance with the '118 patent is shown in FIGS. 4, 5, 6, 7A and 7B in the drawings herein, which correspond respectively to FIGS. 5,3,2,1, and 4 of the '118 patent. Again, for convenience the reference numerals employed in FIGS. 4-7B herein are those appearing in such drawing figures of the '118 patent, to which reference may be had for further details of construction and operation (U.S. Pat. No. 5,200,118 also being incorporated herein by reference).
It will be seen from FIGS. 4-7B herein, and by reference to the specification and claims of the '118 patent, that the choke valve 10 is "divorced" as to its operator control handle 16 and associated linkage from the control handle 28 and associated linkage for the fast idle lever 20, which is thus independently operated through its own crank arm 24 of its bell crank 20. The '118 system thus provides a separate manual control 16 to operate the choke valve 10, and likewise the fast idle latch lever 20 is operated solely by actuating its own control member 28. For convenience to the operator, these two separate actuating members 16 and 28 are associated in their physical location so that they can be easily conjointly manipulated ganged as one unit, if desired, or individually and separately manipulated, as will be seen in FIGS. 4 and 7A.
It will be seen that with the '118 patent system there is no tang coupling between choke lever arm 12 and the fast idle latch bell crank 20 and hence the '118 patent system does not present the aforementioned incomplete choke closure problem of the '480 patent system. This is because the latched-up position of bell crank 20 does not affect or in any way hinder complete closure of choke valve 10, when it is individually manipulated to this condition by its own actuating control 16. Likewise setting bell crank 20 with handle 28 in order to latch up with throttle lever 8 in no way affects choke valve 10. Nevertheless, as in the '480 patent system, when the chain saw engine has been started, and then the throttle trigger depressed, the fast idle lever will be automatically disengaged to allow spring return to its at rest position as shown in FIGS. 4 and 7B.
It should be noted that at some point in time subsequent to the issuance of the '118 patent, a running change was made in the production of carburetors embodying a '118 patent control mechanism. In order to enable setting of the fast idle bell crank latch 20 with the actuating handle 28 adjustably set in a range of "latch-up" positions, several relatively large notches were provided on the free end edge of bell crank arm 22 in place of the single notch 21 referenced in FIG. 4. These notches were designed to be individually engaged by free end edge 23 of throttle lever 8 to set the throttle valve 6 in the fast idle position of FIGS. 5 and 6 regardless of in which of these inner end limit positions the actuating handle 28 was set.
Nevertheless, the aforementioned prior art neither addresses the problems nor provides a solution thereto that insures that, in the case of the '480 fast idle mechanism, as manufactured in mass production practice, the choke will be able to reach the fully closed position at fast idle latch-up. Therefore, the problems of poor starting, or in worst case, "no starting", have continued to prevail for many years despite the wide spread use of the '480 system on carburetors supplied by several major carburetor manufacturers utilizing the '480 system.
These problems resulting from incomplete and/or inconsistent closure of the choke valve in the fast idle starting system of the '480 patent will be better understood by referring to layouts of the choke valve and throttle valve and actuator levers as set forth in FIGS. 8-13 herein.
FIGS. 8, 9 and 10 are vertically arrayed in alignment and illustrate a layout developed in pursuing the invention herein to better analyze the foregoing problems involved in the construction and operation of a commercial embodiment of the '480 fast idle system, wherein parts alike to those in the '480 patent are given like reference numerals. This system layout thus shows throttle valve plate 1, throttle lever 4, fast idle lever 9, choke valve plate 10 and choke lever 12. Throttle plate 1 and throttle lever 4 are mounted on throttle shaft 2 for rotation therewith, and choke lever 12 is mounted on and keyed for rotation with choke shaft 11 for rotating choke plate 10. Fast idle lever 9 is journalled on choke shaft 11 for free rotation relative thereto. Dimensions B, C and D respectively define the width of the carburetor casting body, the center-to-center distance between shafts 2 and 11 and the distance of the center of shaft 2 from the outlet face of the carburetor body.
Dimension E (FIGS. 9 and 12) represents the gap between the free end edge of tang 7 of throttle lever 4 as spaced from surface 8a of notch 8 of fast idle lever 9, with tang 7 resting on face 8b of notch 8 when choke shaft 11 has been rotated by choke lever 12 to the full closed choke position shown in FIG. 9 by manual force operator-applied to the choke operating cable (not shown). FIG. 10 illustrates the position of the parts when operator actuating force is released from choke lever 12 and the parts are allowed to "back up" (retrograde rotation) and thereby assume their fully latched engaged position as held solely by the biasing forces of their respective return springs.
It is to be noted that FIGS. 8, 9 and 10 represent the operation of the parts when manufactured to "nominal" design dimensional specifications, i.e., using the mean dimensional valve of each present production part as presently print specified using the tolerance variation presently allowed in the parts, and thus represents an idealized condition for current production. It will thus be seen that fast idle arm 9 is swung from its rest position in FIG. 8 by control linkage pulling on choke lever 12 to rotate the same counter-clockwise as viewed in FIGS. 8-10. Choke lever 12,, through its engagement with tang 14 of the fast idle lever 9, thus swings lever 9 from the FIG. 8 position counter-clockwise so that the lever free end leading edge 9a, in advance of notch 8, first engages tang 7 of throttle lever 4 prior to notch 8 reaching the FIG. 9 position wherein tang 7, acting as a detent, has sprung into notch 8. Lever 9 continues this counter-clockwise swing through the FIG. 10 position, wherein tang 7 is still detent engaged in notch 8 and is now abutting notch surface 8a, and then completes its operator-driven swing when the parts reach the position of FIG. 9, wherein the corresponding swing of choke valve plate 10 is positively stopped by the protruding portion of its peripheral edge striking the carburetor throat bore surface.
Note that the design layout of FIG. 9 calls for the choke plate 10 being positively stopped in fully closed position at an angle of 15.degree. from a design plane PC that intersects perpendicularly the throat axis X of the carburetor. This interengaged latching position will be achieved by operator manual force applied to the control cable attached to choke lever 12 working against the bias of the return spring (not shown) acting on lever 9, and against the bias of the return spring (not shown) acting on throttle lever 4.
However, note that when the operator releases his control manipulating force, the return springs will retrograde pivot levers 9 and 4 from the FIG. 9 position back to the FIG. 10 position. The FIG. 10 position thus represents the nominal (idealized) fully latched-up condition with the throttle valve plate 1 is solely spring held in fast idle position and the choke valve plate 10 is solely spring-held in nominal fully closed position by the fast idle latch system. It will be seen that the dimension of gap E enables 3.degree. of retrograde pivotal motion of the latch parts from the FIG. 9 to the FIG. 10 position, thereby allowing the return springs to move the throttle valve plate 1 from an inclination of 31.degree. (FIG. 10) to an inclination of 28.degree. relative to a design plane PT coincident with the axis of shaft 2 and perpendicularly intersecting the carburetor throat axis X. More significantly, choke valve plate 10 will swing back open through an angle of 3.degree. from the 15.degree. position shown in FIG. 9 to the 18.degree. inclination position of FIG. 10. However, this FIG. 10 very slightly open position of choke valve plate 10 nevertheless has hitherto been accepted as functionally fully closed for achieving existing carburetor design optimum performance.
FIGS. 11, 12 and 13 are layouts corresponding to FIGS. 8, 9 and 10 respectively and in which the moving parts of the fast idle latch system are laid out on the same scale as FIGS. 8, 9 and 10, but are all theoretically made to one limit of their dimensional tolerances to represent one extreme of the design tolerance stack-up. It will be seen that dimension E in FIG. 12 is substantially greater than the corresponding dimension E in FIG. 9. It will also be seen that the fast idle lever 9 engages tang 7 earlier in its path of swing travel during choke closure, as illustrated by the relative angulation of the parts in FIG. 13 as compared to FIG. 10. Lever 9 finally reaches the stop limit position of FIG. 12 when choke plate 10 is forced against the surface of the carburetor bore in its actual fully closed position, and hence is again inclined at an angle of 75.degree. from the carburetor throat axis X. Then when operator manual force is released from the control actuating member, the biasing forces of the return springs acting on levers 4 and 9 pivot the same back from the position of FIG. 12 to the fully engaged, solely-latch-held position of FIG. 13.
It will be seen that the tolerance stack-up gap E of FIG. 12 thus now enables choke plate 10 to pivot out to a position inclined at 25.degree. from plane PC, which is a full 10.degree. farther open from fully closed position of FIG. 12. Likewise, throttle plate 1 now has pivoted to a fast idle position inclined at 26.degree. from plane PT, which is 2.degree. more closed than the corresponding nominal 28.degree. design position of FIG. 10. Thus allowing choke valve 10 to remain partly so opened, and throttle plate 1 more closed than desired, in their respective latched-up condition causes some level of performance degradation, ranging from starting difficulty to failure to start. Accordingly, inadequate starting A/F enrichment functioning of such valve plates thus plates results when the parts are made to the tolerance stack-up of FIGS. 11-13.
On the other hand, at the other extreme of design tolerance stack-up (not illustrated), the choke valve plate 10 will reach the fully closed stopped position (75.degree. of rotation from fully open) before tang 7 of throttle lever 4 has even engaged any free end edge surfaces of fast idle lever 9. Hence, at this other tolerance limit the result is a complete failure of the fast idle system to function.
By way of example and not by way of limitation, the dimensional values employed for the foregoing analysis illustrated in FIG. 8-13 were as follows(wherein the parts are shown to engineering scale and, for example, dimension B is 33.66 mm in the nominal case):
______________________________________ DIMENSIONAL VALUE NAME OF PART Nominal Worst Case ______________________________________ Width of casting dimension B 33.66 mm 33.28 mm Center-to-Center distance between shafts 24.00 mm 24.12 mm 2 and 11 Dimension D 6.47 6.35 Choke Lever 12 2.62 2.50 Fast Idle Lever 9 3.6 17.45 17.55 55.degree. 56.degree. Throttle Lever 4 7.80 12.83 13.00 Choke Shaft 11 4.72 4.69 2.11 2.06 Choke Shaft Assembly 55.degree. 58.degree. ______________________________________